Conventionally, polymer membranes for water treatment are used for purifying water, for example, for removing turbidity from river water and groundwater, clarification of industrial water, treatment of wastewater and sewage, and as a pretreatment for seawater desalination, and the like.
Usually, such polymer membranes for water treatment are used as separation membranes in water treatment devices that utilize porous hollow fiber membranes formed from various polymer materials such as, for example, polystyrene (PS), poly(vinylidene fluoride) (PVDF), polyethylene (PE), cellulose acetate (CA), polyacrylonitrile (PAN), poly(vinyl alcohol) (PVA), polyimide (PI), and the like. In particular, polysulfone resins are frequently used due to their superior mechanical and chemical properties such as heat resistance, acid resistance, alkali resistance, and the like, and from the additional perspective of the ease of making a membrane.
In general, examples of the properties that are required in a polymer membrane for water treatment, in addition to the goal of separation properties, include having superior water permeability and superior physical strength, high stability toward a variety of chemical substances (namely, chemical resistance), less likelihood of adhesion of impurities during filtration (namely, superior antifouling properties), and the like.
For example, cellulose acetate hollow-fiber separation membranes with a superior balance of mechanical properties and improved water permeability have been proposed (see Patent Document 1).
However, this cellulose acetate hollow-fiber separation membrane has low mechanical strength and its chemical resistance is inadequate. Consequently, there is a problem in that when the separation membrane becomes contaminated, cleaning employs physical means or chemical means using chemical products and is extremely difficult. In addition, due to degradability by microorganisms, it is problematic to use membrane bioreactors (MBRs), the applications of which in sewer water treatment have increased in recent years.
Additionally, polymer membranes for water treatment have been proposed using hollow fiber membranes formed from poly(vinylidene fluoride) resin that have both superior physical strength and chemical resistance (see Patent Document 2). Even when contaminated, these polymer membranes for water treatment can be washed using various chemical agents.
However, poly(vinylidene fluoride) tends to have comparatively little hydrophilic properties, with low antifouling properties.
Furthermore, the use of vinyl chloride resins can be considered for their superior mechanical strength and chemical resistance, but the antifouling properties of vinyl chloride resins are inadequate.
Thus, to improve the antifouling properties of porous membranes using vinyl chloride resins, a type of porous polymer membrane was proposed with a vinyl chloride resin in which a hydrophilic polymer that is a cellulose derivative is coated onto a blended non-woven fabric (see Patent Document 3).
Moreover, a porous membrane was also proposed in which an ethylenediamine-polyoxyalkylene polymer is blended into a vinyl chloride resin (see Patent Document 4).
However, there are problems when blending a hydrophilic polymer, such as that controlling phase separation when a porous membrane is manufactured is difficult, a uniform membrane cannot be obtained, and its performance is not stable.
Thus, membranes for use in water filtration have been proposed to include a comb-shaped polymer from graft polymerization of poly(oxyethylene methacrylate) onto vinyl chloride resin using a copper chloride catalyst coordinated to a tetramine compound (see Patent Document 5).
This reference document describes a polymer blend membrane with a grafted main chain homopolymer and a comb-shaped polymer, with the molecular weight of the comb-shaped polymer grafted side chains being increased to improve membrane hydrophilic properties.
However, compatibility decreased when the molecular weight of the comb-shaped polymer grafted side chains was increased, raising concerns of reduced mechanical strength. In addition, there are concerns about residues from the copper chloride and tetramine compound used in the catalyst.